Method and means for photoelectrostatic charging



July 15, 1969 s. A. GAWRON METHOD AND MEANS FOR PHOTOELECTROSTATICCHARGING Filed Nov. 7. 1966 F. .E. 2H er/M 16% BR MW .5 CS L L m Em WTvm nm We 6 T S T E0 00 NP PD I1\/V1N"/UR STANLEY A. GAWR ON sail WATTORNEY United States Patent 3,456,109 METHOD AND MEANS FORPHOTOELECTRO- STATIC CHARGING Stanley A. Gawron, Mount Prospect, 11].,assignor to Addressograph-Multigraph Corporation, Mount Prospect, Ill.,a corporation of Delaware Filed Nov. 7, 1966, Ser. No. 592,561 Int. Cl.H01t 19/00; G03g 13/02 U.S. Cl. 250--49.5 9 Claims ABSTRACT OF THEDISCLOSURE This invention relates generally to photoelectrostaticimaging, and more particularly, to means for and methods of uniformlycharging a photoelectrostatic member.

In the art of photoelectrostatic imaging, it is conventional toreproduce a graphic subject on a photoelectrostatic member which iscomposed of a photoconductive layer applied to a base support. Theprocess involves imparting a blanket electrostatic charge to thephotoconductive layer through the use of a corona discharge electrode orthe like. This step of charging the photoconductive layer renders itsensitive to radiation of a particular wavelength, generally in theultraviolet and visible wave bands.

While in this sensitized condition, the member is exposed to a radiationpattern produced by illuminating the graphic subject to be reproduced.On such exposure the charge is dissipated in the area where thephotoconductive layer is struck by light, thereby forming anelectrostatic charge pattern. This pattern is then developed by theapplication of an electroscopic powder forming a visible image which maybe fixed by various known techniques.

It is conventional in photoelectrostatic imaging equipment to impart theelectrostatic charge, that is, the charge that sensitizes the member, bypassing the member between two spaced apart opposing corona dischargedevices, each being connected to a DC. corona generating source ofopposite polarity. One of the corona discharge devices emits negativecharges which are conducted to one surface of the member while the otherelectrode produces a positive corona discharge conducting positivecharges to the opposite surface of the member.

The effect of such a charging arrangement is to pass the member betweena high potential field in which the opposite sides of the member areoppositely charged. In this type of operation there results a zeropotential at the interface between the photoconductive layer and thebase support. The same effect can be realized by replacing one of theelectrodes with a conductive support placed against the base support ofthe photoelectrostatic member.

The above-described charging techniques are not entirely satisfactorybecause of the nonuniformity of charge deposition. This is particularlytrue where the photoconductive layer is comprised of finely dividedphotoconductive particles such as zinc oxide dispersed in an insulatingresin binder. However, the problem of nonuniform charge deposition hasbeen observed to occur on other photoelectrostatic members such as theconventional selenium plate.

Another shortcoming of the heretofore known techniques of sensitizingthe photoelectrostatic member resides in the high charge levels appliedto the photoconductive layer because of the high potentials necessary tobe supplied to the emission electrode in order to reach and overcome theionic emission threshold of these corona devices. At the high chargelevels encountered it has been found that the exposure to the lightpattern does not adequately dissipate the high charge and consequentlythe photoconductive layer in the non-image layer will tend to attractelectroscopic material producing what is known as background.

In liquid development systems the latent image-bearing copy sheet isparticularly receptive to the deposition of toner along the edge of thecopy first to be immersed in the developing liquid. The attraction oftoner to the leading edge is believed caused by the copy sheet appearingto have an overall charge polarity the same as the sensitizing chargewhich is opposite in polarity to the particles dispersed in theinsulating liquid. This deposition of toner along the lead portion isundesirable because it obscures the legibility of the information anddetracts from the appearance of the copy.

The use of corona-type discharge devices offers the advantage ofimparting a charge to the surface without having to make physicalcontact with the photoconductive layer.

It is a general object of the" present invention to provide improvedphotoelectrostatic reproductions in which the attraction ofelectroscopic powder is controlled to be uniformly deposited in theimage area and minimizing the deposition in the background.

It is a further object of this invention to provide improved means forimparting a controlled blanket electrostatic charge to aphotoelectrostatic member at a charge level that is optimum forproducing a copy having a uniformly dense image and, at the same time, alow level of background.

It is another object of this invention to provide improved chargingmeans for controlling the net charge applied to the photoelectrostaticmember which charging means is simple in its construction and operation.

In achieving the foregoing objects, there has been provided a modulatingdischarge electrode acting on the photoconductive layer in addition tothe conventional pair of electrodes which are disposed in spaced apartopposing relation to one another forming an accessway through which thephotoelectrostatic member may pass during the charging operation.

The movement of the photoelectrostatic member relative to the pair ofelectrodes produces a first charge on the photoconductive layer and acharge of opposite polarity on the base support. The resulting chargerenders the photoconductive layer sensitive to electromagneticradiation. Immediately after the application of the sensitizing charge amodulating charge is applied to the photoconductive layer which isopposite in polarity to the first charge. The modulating coronadischarge electrode establishes a field between the electrode and thebase support which is at some potential above ground.

A better understanding of the invention may be had from the detaileddescription which follows when read together with the drawing in which:

FIGURE 1 is a schematic of the apparatus embodying this invention;

FIGURE 2 is a perspective view of a charging unit constructed inaccordance with the present invention.

Referring to FIG. 1, there is shown a photoelectrostatic member 10comprising a photoconductive layer 12 applied to a base support 14 beingprocessed through the charging assembly identified generally as 16. Awide variety of materials may be used for the base support material 14such as metal foils, plastic materials, glass and the like. Thepreferred material for the base support is usually paper.

The photoconductive layer 12 may be formed by vacuum deposition ofamorphous selenium onto a metal backing plate or it may comprise finelydivided photoconductor particles such as zinc oxide dispersed in aninsulating resin binder and applied to a paper base. The preparation andformation of photoelectrostatic members is now conventional in the artand need not be treated here.

The photoelectrostatic member is arranged to have relative movement withrespect to the charging assembly 16. The chargeable member may besupported in a fixed position and the charging assembly be made totraverse the length of the member.

For purposes of the instant description the photoelectrostatic memberwill be considered to move relative to the charging assembly it beingunderstood that it is necessary to effect only relative movement betweenthe charging assembly and the member.

The charging assembly 16 is comprised of a pair of primary chargingelectrodes 18 and 20 which are disposed in spaced apart relation oneither side of the path taken by the photoelectrostatic member 10. Thefirst corona electrode 18 is comprised of at least one fine conductivewire 22 which is mounted parallel to the photoelectrostatic member andextends in a direction transverse the direction of movement. Theconductive wire 22 is connected to a high potental D.C. coronagenerating source 22 which generates a high negative potential causingthe wire 22 to emit negatively charged ions to the surface of thephotoconductive layer 12. The wire 22 is surrounded by a conductiveshield 24 which directs the ions in a direction generally perpendicularto the path of travel of the member. The second electrode is similarlycomprised of a fine conductive wire 26 which extends transverse thedirection of movement of the photoelectrostatic member and is likewisecontained in a shield 28. The wire 26 is connected to a D.C. coronagenerating high potential source which generates a positive potentialcausing positive ions to be sprayed on the surface of the base support14. As part of the charging assembly and connected therewith is a thirdelectrode 30 comprised of a fine conductive wire 32 contained in aconductive shield 34 which electrode functions to modulate theelectrostatic charge which has been deposited on the photoconductivelayer 12 by the primary electrode. The fine conductive wire 32 of themodulating electrode 30 is connected to the positive potential source 25so that the photoconductive layer as it passes through the assemblyfirst receives a saturation charge of one polarity and thereafter passesbeneath the modulating electrode to have a charge of lesser potentialthan and opposite in polarity to the charge deposited on thephotoconductive layer 12 by the primary electrode arrangement.

It will be appreciated that the pair of electrodes 18 and 20 whichdeposit the primary electrostatic charge on the photoelectrostaticmember 12 creates a high potential field between the conductive wires 22and 26 which are respectively connected to a negative D.C. coronagenerating source and positive D.C. corona generating source 23 and 25,respectively. In the circumstance that the D.C. generating source is inthe range of 4,000 to 6,000 volts, the wire 22 will carry acorresponding voltage of, for example, minus 6,000, and the wire 26 apotential of plus 6,000, producing a total field of 12,000 volts. Such ahigh field causes the level of charge deposition on the layer 12 torapidly reach a saturation level in the range of 400 to 600 volts.

Understandably, the efficiency of the charging arrangement of theprimary electrodes is desirable since in photocopying procedures it isdesirable that the sensitization step as well as all subsequent steps beaccomplished as efficiently and in as short a time as possible.

The modulating electrode 20 is disposed so that its modulating charge isapplied shortly after the primary sensitizing charge has been depositedtaking its power from the positive D.C. potential source 25. Thiselectrode is intentionally arranged so that the field between the fineconductive wire 32 and the photoelectrostatic member 10 used as thereference for the electrode is of a sufiicient strength to properlymodulate or alter in part the surface charge of the member.Understandably, the degree of modulation may also be regulatedbyincreasing or decreasing the distance between the electrode 30 and thecharged surface of the member 10, by adjusting the potential applied tothe electrode 30, and by varying the speed at which the member is movedpast the electrode. The above factors may be employed to adjust themodulating effect of the electrode 30 without the deleterious effect ofneutralizing the primary charge.

The charge levels necessary to achieve high quality images of properdensity are known to be in the range of 400 to 600 volts, particularlyin the circumstance when the member is of the zinc oxide type. Thischarge level is achieved in fractions of a second as it passes throughthe primary electrode. The measured effect of the modulating electrodeis to neutralize areas of peak charge resulting from the primarycharging and also to generally lower the overall negative charge thathas been applied.

The precise mechanism by which the benefits of the modulating charge arerealized is not fully understood. However, some theories have beenadvanced and their expression here is merely for purposes ofcontributing to a better understanding of the invention without limitingsame.

The photoconductive surface of any photoelectrostatic member, whether itbe produced by the vacuum deposition of metal or by the incorporation ofphotoconductive particles in an insulating resin binder will be somewhatirregular, probably more so in the latter situation, because of theirregularity of the pigment particles. The passage of thephotoelectrostatic members between a pair of spaced apart primaryelectrodes such that the photoconductive layer is exposed to either ahigh negative or high positive potential, the high points of theirregularities on the surface will tend to serve as sites for chargeconcentrations. In other words, a measurement on the surface of thephotoconductive layer after being exposed to a primary charge of about500 volts may vary between different points anywhere from 400 to 600volts. Understandably, this nonuniformity of charge deposition acrossthe surface of the photoconductive member will result in a nonuniformelectrostatic image after it has been exposed to a pattern of light andshadow. Development of such a nonuniform electrostatic image will, ofnecessity, produce a nonuniform material image since the force of theattraction exerted by the areas of charge concentration upon the mass ofelectroscopic powder will be greater than in other areas giving anirregular material image.

As explained earlier, exposure to a light image will cause the chargedareas that are light-struck to be dissipated in those areascorresponding to the background or nonimage portions of the graphicsubject matter. When in the first instance the photoconductive layer ischarged to a high level, exposure to light does not consistentlydischarge the layer in the light-struck areas to a level where therewill not be attraction of electroscopic particles. It is suggested herethat a particular photoelectrostatic member has a threshold voltagebelow which electroscopic powder will not adhere to its surface. Thesevoltages are usually very nominal and for the most part are less than 50volts.

Thus, the application of a modulating voltage to the primary sensitizingvoltage, that is, one that is opposite in sign and which does notoperate as efficiently to deposit its charge on the photoelectrostaticmember accomplishes the following:

(1) Neutralizes peak concentrations of charge on the photoelectrostaticmember to correct for uneven attraction of electroscopic powder to theimage area,

(2) Has an overall effect of lowering the primary voltage so that uponexposure to electromagnetic radiation the exposed areas reach a lowercharge level minimizing the attraction of electroscopic powder to thebackground area, and

(3) Causes the photoelectrostatic member to take an overall chargehaving the same polarity as the toner particles, hence repelling tonerfrom the leading edge without interfering with the attractive force thatthe charges in the image areas have for the toner particles.

It has been suggested that the primary charge be reduced in the firstinstance so as to avoid the necessity of the modulating charge. This hasbeen found unsuccessful since the potentials necessary to achieve coronadischarge or ionization of the atmosphere in the vicinity of the coronawire results in charge levels on the photoconductive members that arequite high and do not provide an opportunity for a sufliciently fineadjustment. The photoconductive layer in a dark adapted condition has aresistivity in the range of to 10 ohm centimeters so that it readilyaccepts a charge. In the charged condition, that is, after itimmediately leaves the primary electrode, its resistivity is in therange of 10 to 10 ohm centimeters. It will readily be appreciated thatwhen it has passed out of the range of the primary electrode theelectrical environment has changed to one of greater resistivity so thatits capacity to accept a charge is substantially decreased. Hence,electrodes which emit charged ions acting into a high resistance mediumwill reduce the current flow and substantially curtail the rate ofcharging as compared to the application of the primary charge.Accordingly, the photoelectrostatic member with its higher resistivityand the electrode 30 operating against a highly insulating surface tendsto result in producing a self-regulating condition in which themodulating charge gives just the precise effect.

Referring now to FIGURE 2, there is illustrated an embodiment of thecharging device according to this invention. The fine conductive wiresare represented by the numerals 22, 26 and 32 and correspond to thewires shown in the electrodes 18, and 30 illustrated in FIG- URE 1. Theelectrodes shown in the embodiment of FIG- URE 2 employ multiple strandsof conductive wires for each electrode.

The charging assembly 16 is formed by mounting the electrodes in a frame40 made of an insulating material such as an acrylic polymer, with theprimary electrodes 42 and 44 arranged opposite one another and themodulating electrode 46 located adjacent to and spaced from theelectrode 42 so as to provide a dielectric medium therebetween. Theconductive wires 22, 26 and 30 are stretched inside the conductiveshields 48, 50 and 52 being secured to the shield structure byinsulating connectors 54. The conductive shields are essentially of ashallow box-like construction enclosing the wires on five sides with oneface of the enclosure being open. The electrodes are positioned with theopen faces parallel to the surface of the photoelectrostatic member 10so that the discharge from the corona wires is generally perpendicularto the path of travel of the member 10 through the charging assembly.

Over the open face of each of the shield structures there is provided anupper and lower support member 60 and 62, respectively, for maintainingthe member 10 in a supported position, out of contact from either of theenergized conductive wires and guiding the sheet along a path uniformlydistant from each of the electrodes. The support members 60 and 62comprise frames 64 over which are laced strands of an insulatingfilament material 66 such as a polyamide plastic or fluorocarbon plasticmaterial. The lacings 66 are stretched over the frames 64 so as topresent an oblique pattern to the direction of the member 10 and notinterfere with the overall deposition of charge.

The electrodes 44 and 46 are connected to a high positive potentialcorona generating power source 68 operating the range of 4 kv. to 6 kv.,preferably about 5 kv., and at a slightly lower potential than the highnegative potential corona generating source 70 which supplies energy tothe electrode 42 and operates at 5.5 to 6 kv., preferably at 5.5 kv. Themodulating electrode 46 is connected to the high positive potentialsource that powers the electrode 44 being supplied a potential of about5 kv. The electrode may be spaced from /2" to 2 /2" from thephotoconductive member 10 at a distance that will lay down a charge lessthan the primary charge. The charge laid down during the time ofexposure is rather small, somewhere in the range of 50-150 volts. Thecopy sheet passes through the electrode assembly at a rate ranging from10 to 30 feet per minue. Passage of the member 10 through the chargingassembly 16 with the electrodes connected to their respective powersupplies as above described results in imparting a primary electrostaticcharge, negative in polarity on the photoconductive layer in the rangeof 400 to 550 volts. The exact saturation charge level which isdeposited by the primary electrodes 42 and 46 will depend on a number offactors such as the kind of photoconductor, the thickness of thephotoconductive layer on the base support, the ratio of thephotoconductive particles to the insulat' ing resin binder in thecircumstance where the photoconductive layer is made up ofphotoconductive pigments, and also in the type of resin binder. Themodulating electrode 46 when connected to a 5 kv. power source willmodulate the primary charge lowering it approximately 10 to 20% from itsoriginal level.

It is to be 'understood that means other than double corona wires may beemployed for imparting the primary sensitizing charge to thephotoconductive layer. For example, the primary sensitizing charge maybe accomplished by roller charging such as disclosed in Tregay et al.US. Patent No. 2,980,834, entitled Charging of PhotoconductiveInsulating Material, or by a single corona wire in conjuction with aconductive surface or a metal plate as a ground plane.

It will be appreciated that the polarity of the modulating potentialmust be opposite to the polarity of the primary potential applied to thephotoconductive layer. Reference in his description to a primary chargeof a negative potential to the photoconductive layer is merely exemplaryand the polarity of the primary charge may be positive as in the case ofsensitizing a selenuim photoelectrostatic member in which case themodulating electrode would then be connected to a high negative coronagenerating power source.

The particular configuration of the electrodes as shown hereinrepresents a preferred embodiment of the invention but it is understoodthat many changes and modifications may be made by one skilled in theart without departing from the essential concept of the invention ascovered in the appended claims.

What is claimed is:

1. A charging assembly for imparting uniformly distributed electrostaticcharges to an electrophotographic member comprising first, second andthird corona discharge electrodes, each including a plurality of spacedparallel, fine conductive wires carried on a non-conductive support,said second electrode mounted adjacent said first electrode and saidthird electrode mounted in opposed spaced parallel relation directlyopposite said first electrode, means for applying a high electricalpotential to said first and third electrodes opposite in polarity, meansfor applying a high electrical potential to said second electrode lowerthan the potential applied to said first electrode and opposite inpolarity, conductive shielding means connected to a reference potentialpartially surrounding the conductive wires whereby the corona emittedfrom said second electrode is of a lesser intensity than the coronaemission from said first electrode without interfering with thesensitizing effect of said first and third electrodes.

7 2. The charging apparatus as claimed in claim 1 wherein said D.C.source supplies a high negative potential to said first electrode and ahigh positive potential to said second and third electrodes.

charges to a photoelectrostatic member having a photoconductive layerapplied to a base support to render said layer responsive toelectromagnetic radiation comprising a pair of corona dischargeelectrodes each including at least one fine conductive wire carried on anonconductive support, said electrodes being located in spaced relationdirectly opposite one another, means for locating saidphotoelectrostatic member between said charging electrodes, a thirdcharging electrode comprising at least one fine conductive wire carriedon a nonconductive support and adjacent one of said pair of electrodesfacing the photoconductive layer, means for moving said membersequentially past said pair of electrodes and said third electrode, ahigh energy source for supplying oppositely poled D.C. potentials tosaid pair of electrodes and a DC. potential to said third electrodepoled oppositely and lower in potential than the potential applied saidadjacent electrode, whereby the intensity of the corona emitted fromsaid third electrode is of a lesser intensity than the emission fromeither of said pair of electrodes without interfering with thesensitizing effect of said paired electrodes.

6. The method of sensitizing a photoelectrostatic member comprising aphotoconductive layer on a base support comprising the steps of:

(a) sensitizing said photoconductive layer by locating the memberbetween a first and second corona discharge electrodes located in spacedrelation directly opposite each other, spraying oppositely poled chargesonto the photoconductive layer and the base support respectively;

(b) spraying another charge onto said layer subsequent to the chargingaction of said second electrode from a third electrode which charge isopposite in polarity to the charge sprayed by said first electrode andof lower potential,

whereby the corona emission from said third electrode is of a lowerintensity than the corona emission from said first electrode.

7. The method of sensitizing a photoelectrostatic member as claimed inclaim 6 wherein said superimposed charge is applied to the chargedportionts of the photoconductive layer concurrent with said portions ofsaid layer passing between said first and second corona electrodes.

8. The method as claimed in claim 6 wherein said first electrodedeposits a high negative charge to the photoconductive surface and saidsecond and said third electrodes apply a high positive potential.

9. In electrostatic photography, means for imparting electrostaticcharges to a photoelectrostatic member having a photoconductive layerapplied to a carrier to establish therein a resistivity in the range of10 -10 ohmcentimeters comprising primary charging electrode means forapplying to the layer a primary sensitizing charge of one polarity andpotential; modulating electrode means adjacent said primary chargingelectrode means for applying to the layer subsequent to the applicationof the primary charge a modulating charge of a polarity opposite that ofthe primary charge, said modulating charge being at a potential lessthan the potential of the primary charge to affect modulation of theprimary charge without altering the sensitizing character of saidprimary charge; means for first locating the member in chargingrelationship with said primary charging electrode means and thenlocating the member in charging relationship With said modulatingelectrode means whereby the corona emission from said modulatingelectrode is of a lower intensity than the corona emission from saidprimary electrode means.

References Cited UNITED STATES PATENTS 2,790,082 4/ 1957 Gundlach250-495 2,965,481 12/1960 Gundlach 961 3,244,083 4/ 1966 Gundlach 951.7

WILIIAM F. LINDQUIST, Primary Examiner US. Cl. X.R. 250

